Maraging steels are carbon-free or low-carbon steels, and are obtained by subjecting steels containing Ni, Co, Mo, Ti and like elements in high proportions to solution heat treatment and then to quenching and aging treatment.
Maraging steels have characteristics including (1) good machinability attributable to formation of soft martensite in a quenched stage, (2) very high strength attributable to precipitation of intermetallic compounds, such as Ni3Mo, Fe2Mo and Ni3Ti, in martensite texture through aging treatment, and (3) high toughness and ductility in spite of its high strength.
Maraging steels have therefore been used as structural materials (e.g. engine shafts) for spacecraft and aircraft, structural materials for automobiles, materials for high-pressure vessels, materials for tools, and so on.
So far, Maraging steels (e.g., 18Ni maraging steels/Fe-18Ni-9Co-5Mo-0.5Ti-0.1Al of Grade 250 ksi (1724 MP)) having high strength and excellent toughness and ductility, have been used for engine shafts of aircraft. However, with the recent demand of improving air pollution by, for example, tightening control on exhaust gas emission, enhancement of efficiency has been required of aircraft also. From the viewpoint of designing engines, there have been increasing demands for high-strength materials capable of enduring high power, downsizing and weight reduction.
As a material having more excellent characteristics than the maraging steels of Grade 250 ksi, there exists GE1014 (tensile strength: on the order of 2,200 MPa) developed by General Electric Company (Patent Document 1), but much higher strength (tensile strength: 2,300 MPa or higher) has been thought to be necessary.
In addition, low-cycle fatigue characteristics are also important, and control of inclusions becomes important because the starting point of fatigue fracture is governed by the chemical species and shape of inclusions.
In order to solve this problem, various suggestions have been offered.
For example, Patent Document 2 has disclosed a steel containing 0.18 to 0.30 weight % of C, 5 to 7 weight % of Co, 2 to 5 weight % of Cr, 1 to 2 weight % of Al, 1 to 4 weight % of Mo+W/2, at most 0.3 weight % of V, at most 0.1 weight % of Nb, at most 50 ppm of B, 10.5 to 15 weight % of Ni, at most 0.4 weight % of Si, at most 0.4 weight % of Mn, at most 500 ppm of Ca, at most 500 ppm of rate earth elements, at most 500 ppm of Ti, at most 200 ppm of O, at most 100 ppm of N, at most 50 ppm of S, at most 1 weight % of Cu, and at most 200 ppm of P, with the balance being Fe and inevitable impurities.
The material disclosed in the document cited above has a high strength on the order of 2,300 MPa. However, it has a high Ni content of 10.5 weight % or more and a low Co content of 7 weight % or less, and hence it still has room for further improvement in strength. In addition, the material disclosed in the document cited above is an Al-added steel, and therefore it is conceived that AlN is formed as an inclusion which affects low-cycle fatigue characteristics and carries a potential for deterioration of low-cycle fatigue characteristics.
Patent Document 3 has disclosed a maraging steel containing 0.10 to 0.30 mass % of C, 6.0 to 9.4 mass % of Ni, 11.0 to 20.0 mass % of Co, 1.0 to 6.0 mass % of Mo, 2.0 to 6.0 mass % of Cr, 0.5 to 1.3 mass % of Al, and at most 0.1 mass % of Ti, with the balance being Fe and inevitable impurities, and besides, which satisfies a relational expression 1.0≤A value≤1.08.
The material disclosed in the document cited above has a high tensile strength on the order of 2,400 MPa, but it is an Al-added steel as is the case with the material disclosed in Patent Document 2. Thus it is conceived that AlN is formed as an inclusion which affects low-cycle fatigue characteristics and carries a potential for deterioration of low-cycle fatigue characteristics.
Further, Patent Document 4 has disclosed an age hardenable martensitic steel containing 0.21 to 0.34 weight % of C, at most 0.20 weight % of Mn, at most 0.1 weight % of Si, at most 0.008 weight % of P, at most 0.003 weight % of S, 1.5 to 2.80 weight % of Cr, 0.90 to 1.80 weight % of Mo, 10 to 13 weight % of Ni, 14.0 to 22.0 weight % of Co, at most 0.1 weight % of Al, at most 0.05 weight % of Ti, at most 0.030 weight % of Ce, and at most 0.010 weight % of La, with the balance being Fe.
The material disclosed in the document cited above has a high strength of at least 2,300 MPa. However, such a material has not undergone addition of Al, and there is no factor for strength boost resulting from precipitation of a NiAl phase. Such being the case, addition of Ni in an amount of 10 weight % or more is not commensurate with various characteristics attained. Furthermore, the material disclosed in the document cited above contains Ce and the like for the purpose of inhibiting sulfide formation, on the contrary, there is a fear that the sulfides of those elements will form a starting point and easily induce fatigue fracture.
Patent Document 1: U.S. Pat. No. 5,393,488
Patent Document 2: U.S. A-2008/0193321
Patent Document 3: JP-A-2014-12887
Patent Document 4: U.S. Pat. No. 5,866,066